[from archaic English verb for shudder, as with fear] The grue was originated
in the game Zork (Dave Lebling took the name from
Jack Vance's Dying Earth fantasies) and used in
several other Infocom games as a hint that you
should perhaps look for a lamp, torch or some type of light source.
Wandering into a dark area would cause the game to prompt you, “It is
very dark. If you continue you are likely to be eaten by a grue.”
If you failed to locate a light source within the next couple of moves this
would indeed be the case.

The grue, according to scholars of the Great Underground Empire, is a
sinister, lurking presence in the dark places of the earth. Its favorite
diet is either adventurers or enchanters, but its insatiable appetite is
tempered by its extreme fear of light. No grues have ever been seen by the
light of day, and only a few have been observed in their underground
lairs. Of those who have seen grues, few have survived their fearsome jaws
to tell the tale. Grues have sickly glowing fur, fish-mouthed faces, sharp
claws and fangs, and an uncontrollable tendency to slaver and gurgle. They
are certainly the most evil-tempered of all creatures; to say they are
touchy is a dangerous understatement. “Sour as a grue” is a
common expression, even among grues themselves.